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Ind. Eng. Chem. Res. 2000, 39, 3038-3041
Concentration and Purification of Fluosilicic Acid by Membrane Distillation M. Tomaszewska† Institute of Inorganic Chemical Technology, Technical University of Szczecin, ul. Pulaskiego 10, 70-322 Szczecin, Poland
The process was carried out in a plate and frame module with a poly(vinylidene fluoride) membrane. Experiments were performed with the feed and permeate temperature kept respectively at 333 K (or 343 K) and 293 K. During membrane distillation (MD) of fluosilicic acid solutions, the permeate flux declined from 385 to 11.4 dm3/(m2 d) with an increase of acid concentration in the feed from 2 to 35 wt %, at TF,in ) 343 K. The retention coefficient was close to 92%. A combination of membrane distillation with osmotic membrane distillation was also studied. In this case a saturated brine was used as the cold solution. Under these conditions the driving force (partial pressure difference) was higher than that for pure MD. MD experiments of a solution containing both fluosilicic and phosphoric acids, with the acid concentration of 6% H2SiF6 and 30% P2O5, respectively, were also performed. At such conditions the water vapor as well as volatile fluorine compounds are transported through a hydrophobic membrane from the feed solution to the cold distillate. The distillate was then a fluosilicic acid, practically free of phosphates. The investigations with a raw technical fluosilicic acid were also performed. Introduction The production of phosphoric acid by a wet process involves the problem of disposing of fluorine compounds, which are potential environmental pollutants. Fluoride scrubbers are generally used in a modern plant,1-4 and the resulting fluosilicic acid liquor is recovered for sale or as a byproduct.1 It is used for water fluoridation and as an intermediate in the production of various fluorine derivatives, such as fluosilicates, cryolite, and aluminum fluoride. Sodium fluorosilicate produced in greater amounts than all other fluosilicates is one of the cheapest fluorine carriers. The largest current uses are for water fluoridation and in the production of synthetic kryolite.4 Membrane distillation (MD) is the process where evaporation of volatile compounds occurs through pores of a microporous hydrophobic membrane. The driving force of MD is the vapor pressure difference caused by existing temperature and composition gradients between the two aqueous solutions, separated by a hydrophobic membrane.5-8 The separation mechanism of the MD process is based on the vapor/liquid equilibrium of the liquid mixture.9 The advantage of MD among other processes is the possibility of the concentration of solutions even to the supersaturated state.10,11 Furthermore, the permeate obtained is high-purity water for a feed containing nonvolatile solutes. Moreover, the process can be performed at a feed temperature considerably below its boiling point.12 This permits the utilization of a waste heat existing at various industrial sites. Osmotic membrane distillation (OMD) is the process where the driving force is a partial pressure gradient induced by a difference of the solution concentrations on both sides of a hydrophobic membrane. Saturated NaCl solution is often used for decreasing vapor partial pressure on the permeate side.13 † Tel.: +48 91 4494367. Fax: 48 91 4330352. E-mail:
[email protected].
The aims of the present work were to attempt application of membrane distillation for the concentration and separation of fluosilicic acid and studies on a combination of MD and OMD for its concentration. Methods The process was studied in a plate-and-frame module with Durapore membranes (Millipore) made from poly(vinylidene fluoride). The nominal pore size of the membrane was 0.45 µm and the porosity 77%. The laboratory setup for direct contact membrane distillation studies is presented in Figure 1. The main element of the system was the plate-and-frame module consisting of the two compartments, warm and cold, separated by the hydrophobic membrane. The effective membrane area was 104.72 cm2. The warm feed and cold distillate were circulated countercurrently in the closed thermostated systems. Counter-current flow is more efficient but larger trans-membrane pressures are created in this case. A problem due to increased ∆P values could arise if they become higher than the liquid entry pressure (LEP) of the membrane. Our previous unpublished results pointed out that co-current flow of the streams is efficient because this can avoid the module degassing problem. Fluosilicic acid solutions of various initial concentrations were used as the feed (initial volume, 0.500 dm3). The initial volume of the crude fluosilicic acid used as the feed was 1.500 dm3. The separation of fluorine compounds from the acid solution was carried out from a solution containing fluosilicic and phosphoric acids with initial volumes of 0.750 dm3. The vapor transported through the pores of the membrane was condensed directly in the solution in the cold compartment (called “distillate”). The stream flowing through the pores of a membrane is called the “permeate”. The cold system was initially supplied with distilled water in the MD case (the initial volume, 0.400 dm3) or a saturated aqueous NaCl solution (the initial
10.1021/ie9908534 CCC: $19.00 © 2000 American Chemical Society Published on Web 07/11/2000
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Figure 1. Diagram of the MD setup: (1) MD module, (2) membrane, (3) thermostat, (4) heat exchanger, (5) manometer, (6) deaerator, (7) pump, (8) feed tank, (9) distillate tank, and (10) thermometer.
volume was the same, 0.400 dm3) when a combination of MD and OMD was used. The experiments were carried out with the inlet temperature of the feed and distillate (or saturated brine) kept respectively at 333 or 343 K and 293 K. The feed outlet temperatures decreased to 329 or 337 K, respectively. Outlet temperatures in the distillate increased to 297 or 299 K. The observed changes of the feed and distillate temperatures were the effect of heat transport through a membrane.7,8,14 The linear velocity of warm and cold solutions was maintained at 3 cm/s (laminar flow, Re ) 120-260). Changes in distillate volume and the changes in concentration in the feed and distillate were measured systematically every hour, unless otherwise reported. The real concentration of fluorine compounds in the permeate and the molar flux of fluorine compounds calculated as H2SiF6 were estimated from the material balance, taking into account the changes in volume and concentration in the distillate,
NH2SiF6 )
(ct+1Vt+1 - ctVt)24 (mol/(m2 d)) AMt
(1)
where ct and ct+1 are the distillate concentrations at time t and t + 1, Vt and Vt+1 are the volumes of the distillate at time t and t + 1, A is the working area of the membrane, M is the molar weight of H2SiF6, and t is the time between the measurements. The retention coefficient (R) of the acid in a feed was calculated according to the expression
R)
c F - cP × 100 (%) cF
(2)
where cF is the acid concentration in the feed and cP is the acid concentration in the permeate. Results and Discussion The results of MD experiments performed by the use of fluosilicic acid solutions at different initial concentrations in the feed are presented in Figure 2. Because of the exponential rise in the vapor pressure curve, the permeate flux was increased with an increase in the feed temperature, from 280 dm3/(m2 d) at 333 K to 385 dm3/ (m2 d) at 343 K, whereas at 0% H2SiF6 (water) permeate flux was 288.0 or 424.8 dm3/(m2 d), respectively. With increasing acid concentration in the feed from 2 to 35 wt %, the permeate flux declined from 384.0 to 12.0 dm3/
Figure 2. Influence of fluosilicic acid in a feed on the permeate flux during the concentration by MD.
Figure 3. Membrane distillation of a crude fluosilicic acid.
(m2 d). This dependence results from the decrease in the equilibrium water vapor pressure of the solution, with increasing solute concentration in the feed. The retention coefficient of the acid was about 92% for 20% H2SiF6 used as a feed. When the acid concentration in the feed was higher than 20%, the retention of H2SiF6 was lower. This was a result of the increasing vapor pressure of HF and SiF4 above the fluosilicic acid solution at higher concentration.2 H2SiF6 does not exist in the vapor phase.3 The fluosilicic acid under the influence of heat decomposes into volatile HF and SiF4. At these conditions both water and volatile species of fluorine are transported from the warm feed through pores of a hydrophobic membrane to the distillate, where they reform H2SiF6. The concentration of a crude acid from the Chemical Plant “Police” at an initial concentration equal to 14 wt % was increased to 24 wt % H2SiF6. As can be seen from Figure 3, the volumetric permeate flux was lower than that for the model solutions, presented in Figure 2, and the retention coefficient of the acid decreased to 40%. This can be due to the presence of different species dissolved in the crude acid. The crude acid of concentration in the range of 1316 wt % H2SiF6 can contain the following: P2O5, 0.0%50.1%; Fe, 0.001%-0.005%; Al, 0.0003%-0.001%; Ca, 0.001%-0.01%; Mg, 0.0003%-0.002%; SO42-, 0.05%0.3%; Cl-, 0.14%-0.3%. According to Raoult’s law, the salt presence in the solution decreases the water partial pressure, thereby lowering the permeate flux. During the concentration, dissolved species are retained in the feed and can act as salting out agents for the other volatile species.15 Thus, more H2SiF6 (in the form of HF and SiF4) is transported through pores of the membrane in comparison with the model solution.
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Figure 4. Dependence of fluosilicic acid concentration in a feed on permeate flux during the concentration by membrane distillation and osmotic membrane distillation.
The combination of membrane distillation with osmotic membrane distillation was also studied, taking into account mainly the concentration of fluosilicic acid. The saturated brine was then placed into the permeate chamber. As mentioned earlier, fluosilicic acid is used for sodium fluosilicate production. The salt can be easy precipitated from fluosilicic acid by means of saturated NaCl solution. This leads to the idea of combining MD and ODM, where a part of the volatile fluorine compounds transported through the membrane during the concentration would react with NaCl, precipitating Na2SiF6, giving the useful product Na2SiF6. In these conditions (salt-saturated solution in cold compartment) the driving force of water vapor transport was higher than that for pure MD. However, the volumetric permeate flux (for 2% acid used as the feed) increased by a small amount from 268.8 to 302.4 dm3/(m2 d) at 333 K. The presence of brine in the cold distillate really induced precipitation of sodium fluosilicate in the solution, but also on the membrane surface, especially at a high fluosilicic acid concentration in the feed. The precipitate created directly on the membrane surface obstructed pores of the membrane and decreased permeate flux through the membrane, faster than that for MD only, as shown in Figure 4. X-ray diffraction showed the precipitate to contain sodium fluosilicate with a small content of hydrated sodium silicate. The presence of concentrated acids, such as phosphoric or sulfuric, in the feed containing fluosilicic acid increases the partial pressure of fluorine compounds.1 This effect was utilized for the separation of H2SiF6 from the model solution containing the acids fluosilicic and phosphoric of 6% H2SiF6 and 30% P2O5. At such conditions water vapor, HF, and SiF4 are transported through a hydrophobic membrane from the feed solution to the cold distillate. The HF/SiF4/H2O ratio of the vapors depends on the fluosilicic and phorsphoric acids concentrations in the feed. In the presence of a weak (2832% P2O5) phosphoric acid, silicon tetrafluoride can be preferentially volatilized because its vapor pressure is higher than that of HF.1,16 However, more and more hydrogen fluoride escapes as the P2O5 concentration in the feed increases. The fluorine vapors are absorbed in the cold distillate to give a fluosilicic acid. At molar ratios HF/ SiF4 in the permeate below 2 (stoichiometric ratio), the silicon tetrafluoride hydrolyzes forming fluosilicic acid and silica.17 Therefore, the distillate besides fluosilicic acid and water can contain silica. Phosphoric acid is a nonvolatile compound and it remains in the feed. Therefore, the concentration of phosphoric acid
Figure 5. Changes of flousilicic acid concentration and P2O5 in the feed permeate during separation fluorine compounds from the model solution by MD.
Figure 6. Changes in flousilicic acid and P2O5 concentration in the feed permeate during separation of fluorine compounds from a crude acid.
increases during MD and fluosilicic decreases in the feed (Figure 5). At the same time, H2SiF6 concentration in the permeate increases gradually. At 40 wt % P2O5 in the feed, water vapor pressure is very low18 and the volume of the permeate flux is decreased to 12.0 dm3/ (m2 d), but the separation of volatile fluorine compounds increased and their flux through the membrane (calculated as H2SiF6 molar flux) reached 48.2 mol/(m2 d). Because the retention coefficient of phosphoric acid was above 99.9%, the distillate was essentially free of phosphates. The results of MD investigations with a crude fluosilicic acid are presented in Figure 6. The process run was similar to that for the model solution, but the volumetric permeate flux was lower and decreased from 33.6 to 3.6 dm3/(m2 d). This could be affected by the presence of other dissolved species in the raw acid, which decreased the water partial pressure. The molar flux of volatile species is calculated from their changes in concentration in the distillate and volumetric permeate flux (eq 1). Despite the low volumetric permeate flux, the flux of fluorine compounds, calculated in terms of H2SiF6 moles rose to 79.2 mol/(m2 d). With an increase in phosphoric acid concentration, the fluosilicic acid content in the feed was decreased from 6 to 2 wt %. Figure 6 shows that the concentration of H2SiF6 in the distillate was higher than that in the feed and achieved 7 wt %. Under MD conditions impurities of the crude fluosilicic acid are retained in the feed. As a result of the separation of the fluorine compounds during MD, the product was pure fluosilicic acid, practically free of phosphate.
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Conclusion The MD process could be applied for concentrations up to 20% of fluosilicic acid. The combination of membrane distillation and osmotic membrane distillation cannot be used for concentration of the acid, because of the precipitation of sodium fluosilicate directly on the membrane surface and pore blocking of the hydrophobic membranes. With a feed containing fluosilicic acid and phosphoric acid, MD can separate these components, generating a permeate, practically free of phosphate, with a higher H2SiF6 concentration than that in the feed. Literature Cited (1) Slack, A. V., Ed. Phosphoric Acid; Marcel Dekker: New York, 1989. (2) Bidder, J.; Hallsworth, J. A. Two Processes Recover Fluosilicic Acid in Useful Concentrations. Phosphorus Potassium 1974, 73, 35. (3) Becker, P. Phosphates and Phosphoric Acid; Marcel Dekker: New York, 1989. (4) The Chemistry and Chemical Technology of Fluorine. In Encyclopedia of Chemical Technology; Kirk-Othmer, Ed.; John Wiley & Sons. Inc.: New York, 1996; Vol. 9. (5) Lawson, K. W.; Lloyd, D. R. Membrane Distillation. J. Membr. Sci. 1997, 124, 1. (6) Drioli, E.; Wu, Y.; Calabro, V. Membrane Distillation in the Treatment of Aqueous Solutions. J. Membr. Sci. 1987, 33, 277. (7) Schofield, R. W.; Fane, A. G.; Fell C. J. Gas and Vapour Transport through Microporous Membranes. II. Membrane Distillation. J. Membr. Sci. 1990, 53, 173.
(8) Schofield, R. W.; Fane, A. G.; Fell, C. J.; Macoun, R. Factors Affecting Flux in Membrane Distillation. Desalination 1990, 77, 279. (9) Mulder, M. Basic Principles of Membrane Technology; Kluwer Academic: Dordrecht, 1991. (10) Tomaszewska, M. Concentration of the Extraction Fluid from Sulfuric Acid of Phosphogypsum by Membrane Distillation. J. Membr. Sci. 1993, 78, 277. (11) Wu, Y.; Kong, Y.; Liu, J.; Zhang, J.; Xu, J. An Experimental Study on Membrane DistillationsCrystallization for Treating Waste Water in Taurine Production. Desalination 1991, 80, 235. (12) Schneider, K.; van Gassel, T. J. Membrandestillation. Chem.-Ing.-Tech. 1984, 56, 514. (13) Sheng, J.; Jonson, R. A.; Lefebvre, M. S. Mass and Heat Transfer Mechanisms in the Osmotic Membrane Distillation Process. Desalination 1991, 80, 113. (14) Tomaszewska, M.; Gryta, M.; Morawski, A. W. Study on the Separation by Direct-Contact Membrane Distillation Process. Sep. Technol. 1994, 4, 244. (15) Tomaszewska, M.; Gryta, M.; Morawski, A. W. The Influence of Salt in Solution on Hydrochloric Acid Recovery by Membrane Distillation. Sep. Purif. Technol. 1998, 14, 183-188. (16) Fluorine Recovery in Phosphoric Acid Plants. Phosphorous Potassium 1987, 152, 36. (17) Denzinger, H. F. J.; Ko¨nig, H. J.; Kru¨ger G. E. W. Fluorine Recovery in the Fertilizer IndustrysA Review. Phosphorous Potassium 1979, 103, 33. (18) Tomaszewska, M.; Gryta, M.; Morawski, A. W. Study on the Concentration of Acids by Membrane Distillation. J. Membr. Sci. 1995, 102, 277.
Received for review November 24, 1999 Revised manuscript received April 27, 2000 Accepted May 7, 2000 IE9908534
